1. Field of the Invention
The present invention relates to a valve operating mechanism for an internal combustion engine. More particularly, the present invention pertains to a variable valve mechanism wherein the lift and/or the valve opening period can be changed in accordance with the engine operating conditions.
2. Description of the Prior Art
In internal combustion engines, it has been well known that the overlap period between intake and exhaust valves be as small as possible under a low speed engine operation for the purpose of decreasing the suck back of the exhaust gas and the blow back of the intake mixture which may otherwise cause combustion instability and poor fuel economy. For the purpose, it is advisable to make the valve lift and the valve open period as small as possible. However, under a high speed engine operation, it is desirable to increase the valve lift and the valve open period so that an adequate intake mixture change can be provided for high output engine operations.
To comply with such demand, it has already been proposed to provide a plurality of valve actuating cams of different configuration so that one of such cams is selectively used in accordance with the engine operating condition to provide a desired valve operation. For example, Japanese utility model application No. 56-69407 filed on May 15, 1981 and disclosed for public inspection on Nov. 18, 1982 under the disclosure No. 57-182205 discloses a variable valve actuating mechanism including a rocker arm swingable about a rocker shaft and a pair of valve actuating cams, one for high speed engine operation and the other for low speed engine operation. The rocker arm is provided at one end with a first tappet engaging surface which is held in contact with the low speed cam and with a second tappet engaging member for engagement with the high speed cam. The second tappet engaging member is mounted slidably on the rocker arm and a stopper is provided for restricting the sliding movement of the second tappet engaging member when the stopper is in the operative position. In order to move the stopper between the operative position and an inoperative position, an actuator is provided.
It will therefore be understood that, when the stopper is in the inoperative position, the second tappet engaging member is freely slidable so that the low speed cam functions to operate the rocker arm. When the stopper is actuated to the operative position, however, the second tappet engaging member is locked against the slidable movement so that the high speed cam functions to operate the rocker arm to thereby increase the valve lift and the valve open period. It should be noted however that it is unavoidable to provide a certain extent of gap between the second tappet engaging member and the rocker arm in order to allow the aforementioned sliding movement of the second tappet engaging member. Such gap will then produce a sideward play of the second tappet engaging member under a side thrust which will possibly be given by the high speed cam to the second tappet engaging member. Thus, noise may possibly be produced and wear of the sliding surfaces will be accelerated. Further disadvantages in this type of mechanism are that the mechanism is complicated and the inertia of the rocker arm is increased due to the complicated mechanism provided mostly on one end of the rocker arm. Such increase in the inertia of the rocker arm may sometimes cause a jumping of the valve under a high speed engine operation so that the top speed of the engine may have to be decreased.
In the U.S. Pat. No. 4,448,156 which corresponds to the published French patent application No. 2,493,915 and the European patent publication No. 52,554, there is disclosed a valve actuating mechanism including a pair of cams of different shape which are respectively engaged with cam engaging rocker arms swingably mounted on a common rocker shaft. Between the cam engaging rocker arms, there is a valve engaging rocker arm which is mounted at one end swingably on the common rocker shaft. Between the valve engaging rocker arm and the cam engaging rocker arms, there are respectively provided dog clutch devices so that the valve engaging rocker arm is engaged alternately with either one of the cam engaging rocker arms to swing therewith as a unit. The valve engaging rocker arm is mounted on the rocker shaft for slidable movement along the axis of the rocker shaft to be alternately engaged with one of the cam engaging rocker arms. It should however be noted that the mechanism proposed by the U.S. patent is disadvantageous in that the valve engaging rocker arm must be offset with respect to the valve tappet.
A further disadvantage common to the aforementioned known structures is that the valve operating force is transmitted from the cams to the rocker arm in an offset manner so that a smooth movement of the rocker arm is disturbed and wear of the rocker arm is accelerated. In the conventional arrangements, a pair of cams are provided in a side-by-side relationship, one for the low speed operation and the other for the high speed operation so that the valve operating force is transmitted to the rocker arm always from either one of the cams, so that the aforementioned problems are produced.